sensors-detect: Introduce automatic mode

Add option --auto to sensors-detect, for non-interactive use. Default
answer is assumed to every question.
sensors-detect: Detect incorrect non-interactive runs

It is better to use option --auto for non-interactive runs of the
sensors-detect script than faking an input to get the default answers
to all questions.
sensors-detect: Rework option handling

Rework the way command line options are handled. If we ever need more
than this, we should consider moving to some GetOpt perl module.
Extend the documentation of sensors_init() and
sensors_get_adapter_name(). Paul Crawford asked questions about these
as he found the original documentation unclear. I'm including my
answers to his questions here, reformatted, so that other developers
can benefit from them too.
sensors-detect: Fix use of uninitialized value 'vendor_id' on ppc64

Based on a report and an original patch from Jaromir Capik:

There's no vendor_id entry in the /proc/cpuinfo file in case of PPC64
systems and consequently the script returns 3 screens full of the
following errors:

Use of uninitialized value in string eq at /usr/sbin/sensors-detect
line 6444
Use of uninitialized value in string eq at /usr/sbin/sensors-detect
line 6459
Use of uninitialized value in string eq at /usr/sbin/sensors-detect
line 6473
sensors-conf-convert: Add a manual page

Contributed by Jaromir Capik.